In troll fishing, it is desirable to maintain one's lure or bait at a constant depth, and to be able to repeat this depth once it is determined how deep the fish are located. It is also desirable that the fishing line descend from the boath at a steep angle, to minimize line drag and the changes of entangling the line with obstacles, other lines, and the like. While at trolling depth, the trolling device should be stable; that is, it should not veer from side to side or vary in depth. Furthermore, it should not flutter or otherwise produce distracting noise or movement.
Prior U.S. patents disclose devices in the nature of hydrodynamic plates designed to be towed through water by a boat. These devices are generally intended to maintain a bait or lure line at trolling depth; to do so, they may be constrained to have a certain angle of attack to the water. U.S. Pat. No. 2,645,053 discloses an exemplary prior art device, which trails behind the boat at an angle of about 15 degrees to horizontal.
To our knowledge, none of the many prior art proposals has achieved marked commercial success. The reason for this is a matter for conjecture; perhaps the prior devices lacked adequate stability or depth predictability, or perhaps they could not obtain sufficiently steep depression angles.
Blevins' Patent, No. 4,546,565, discloses a trolling device which improved over the prior art in terms of stability and depression angle. The device now disclosed is a further improvement, in that it allows the device to tilt to an inactive, horizontal position either when a fish strikes a line attached to the device, or when an obstacle on the river bottom is encountered.